1. Field of the Invention
Embodiments of the invention relate to a liquid crystal display capable of reducing a distortion of a common voltage.
2. Discussion of the Related Art
Active matrix type liquid crystal displays display a moving picture using a thin film transistor (TFT) as a switching element. The active matrix type liquid crystal displays have been implemented in televisions as well as display devices in portable devices, such as office equipment and computers, because of the thin profile of an active matrix type liquid crystal displays. Accordingly, cathode ray tubes (CRT) are being rapidly replaced by active matrix type liquid crystal displays.
In the active matrix type liquid crystal displays, a data voltage is applied to a pixel electrode, and a common voltage is applied to a common electrode opposite the pixel electrode. The common electrodes are connected in parallel to common lines. Liquid crystal cells are driven by the voltages applied to the pixel electrodes and the common electrodes.
However, the common voltage is easily distorted by a resistance of the common line or a deviation of the common voltage over the entire surface of a liquid crystal display panel depending on a structure of the common line. For example, in a liquid crystal display in which as many common lines as the number of horizontal lines (i.e., a vertical resolution) are formed parallel to gate lines, because a data voltage is simultaneously applied to pixels of 1 horizontal line through the supply of scan pulses, a load of the common line opposite the pixels increases. Because the load of the common line depends on an amount of RC delay defined by a multiplication of a resistance and a parasitic capacitance of the common line, the resistance of the common lines has to be reduced so as to reduce the amount of RC delay. However, as shown in FIG. 1, because a related art liquid crystal display has a structure to receive a common voltage Vcom through only two input units, it is limited to a reduction in resistances of common lines. As a result, in the related art liquid crystal display, as shown in FIG. 2A, the common voltage Vcom is not kept constant and is affected by a scan pulse SP or a data voltage Vdata. Hence, a ripple phenomenon is generated in the common voltage Vcom. The ripple of the common voltage Vcom is a main cause generating a horizontal crosstalk when a specific data pattern is displayed on the screen as shown in FIG. 3A.
In the related art liquid crystal display, the resistance of the common line increases because of the structure of the common line shown in FIG. 1, as the common line goes from right and left sides to a middle portion of the liquid crystal display panel. Therefore, as shown in FIG. 2B, a deviation of the common voltage Vcom over the entire surface of the liquid crystal display panel is caused. The deviation of the common voltage Vcom, as shown in FIG. 3B, causes a luminance difference between upper and lower portions of the liquid crystal display panel and a flicker, and also accumulates a DC component inside the panel to cause image sticking. In most of liquid crystal displays, common lines formed at edges (i.e., a non-display area outside a pixel array) of a panel have wide width so as to reduce resistances of the common lines. However, it is limited to a reduction in the resistances of the common lines because of the limited size of the non-display area. Further, there is a relatively large deviation among resistances of the common lines depending on locations of the common lines.
Further, because as many common lines as the number of horizontal lines are needed in the related art liquid crystal display, it is limited to an increase in a luminance by increasing an aperture ratio of a pixel array.